In the field of optically applying a dielectric film, anti-reflection films and infrared reflection films used in the form of a single layer film, and further, anti-reflection films, reflection-increasing films and semi-permeable films allowing specific wavelengths to be reflected or permeate selectively, designed in the form of a multi-layer film of approximately 10 layers, and the like are used.
Recently, with enlargement and development of industries in the optical communications field, optical filters capable of precisely selecting a specific wavelength are desired. For example, among optical filters used in demultiplexer and multiplexer which are essential parts as a module for optical communications of a wavelength multiplex mode are a narrow band filter and edge filter, and dielectric multi-layer films realizing these are generally constructed of a multi-layer film containing from tens to several hundreds or more layers, and the spectral property thereof is optically designed using the refractive index (n) and film thickness of each layer as parameters.
The above-mentioned optical multi-layer film is manufactured by alternately laminating a transparent thin film having a relatively high refractive index and a transparent thin film having a relatively low refractive index. When the difference in refractive index of two kinds of materials laminated alternately is large, design of an optical multi-layer film is more advantageous, since a desired optical property can be realized with a smaller number of layers.
In general, as for the range of a high refractive index of the above-mentioned optical multi-layer film, a range from 1.9 to 2.5 is exemplified, and as a range of a low refractive index thereof, a range from 1.3 to 1.6 is exemplified, conventionally. As the transparent material of a high refractive index, titanium oxide (n≈2.4: n represents refractive index), zinc sulfide (n≈2.3), zirconium oxide (n≈2.2), niobium oxide (n≈2.2), tantalum oxide (n≈2.1), cerium oxide (n≈2.2)m hafnium oxide (n≈2.2), neodymium oxide (n≈2.1), tungsten oxide (n≈2.0), tin oxide (n≈2.0), tin-doped indium oxide (n≈1.9), yttrium oxide (n≈1.9) and the like are known. AS the transparent material of a low refractive index, silicon oxide (n≈1.46), magnesium fluoride (n≈1.38) and the like are known.
The refractive index has wavelength dependency, and the above-mentioned refractive index is a value in the near infrared wavelength range.
An optical multi-layer film using these transparent materials is formed on a substrate by vacuum film-forming methods such as a vacuum evaporation method, sputtering method and the like. Thin film-forming methods are appropriately selected depending on use, necessary properties, productivity and the like.
In the cases of effecting film-forming over a wide range area with uniform thickness, of effecting film-forming at a relatively low temperature, and of raising close adherence with a substrate, a sputtering method is advantageous.
As the high refractive dielectric material, titanium oxide having a high refractive index and easily available from the standpoint of cost is widely used among the above-exemplified compounds. However, it is known that a thin film of titanium oxide generally has the following problems.
(1) Variation in thin film formation conditions (for example, degree of vacuum, oxygen partial pressure and the like in an evaporation method) tends to change the refractive index of the titanium oxide obtained, and reproducibility of the properties of an optical multi-layer film is difficult to obtain. The reason for this is that even if the degree x of oxidation of TiOx varies, stoichiometrically stable TiO2 is not necessarily obtained. As a result, an optical multi-layer film having a designed optical property may not be obtained in some cases.
(2) A film-forming speed in forming a dielectric film by a sputtering method is extremely slow. A radio frequency (RF) sputtering method has been recently used as a general technology for a target material having small electrical conductivity and film-forming speed has been improved, however, the RF sputtering method is still in an industrially disadvantageous condition.
(3) The optical properties of a titanium oxide thin film, particularly, refractive index change over time during storage in air. Therefore, in the case of an optical filter using titanium oxide as a high refractive material, there occurs a problem in that the wavelength to be selectively allowed to permeate and be reflected shifts from its given position.
(4) Film stress is as relatively large as about 1 GPa (compression). On the other hand, the film stress of silicon oxide widely used as a low refractive material is about 0.2 GPa (compression), and this difference produces problems in cracking of a film and peeling of a film at the interface of a multi-layer film.
The problems of the above-mentioned (1) has been attempted to be solved by designs of equipment and operation such as improvement in an apparatus of controlling an atmospheric gas introduced into a vacuum layer, improvement of operation conditions, selection of raw materials, and the like. However, these are only intrinsic improvements in facilities and methods, and not accepted as technologies capable of solving the above-mentioned problem.
Regarding the problem in the above-mentioned (2), Japanese Unexamined Patent No. Hei-7-233469 discloses, for example, that film-forming speed can be improved by a DC sputtering method in which an oxide sintered body target of TiOx is allowed to have electrical conductivity. Also, addition of other oxides for increasing film-forming speed is disclosed. Further, Japanese Unexamined Patent No. Hei-8-283935 discloses that film-forming speed can be improved by using a target having a smaller degree of oxidation than that of the stoichiometrical composition and minimizing an oxygen gas introduced.
However, these technologies are only related to film-forming speed. Additionally, since an oxide or suboxide target is used, there is another problem in that coincidence of the composition of a target and the composition of the resulting oxide thin film is difficult.
Regarding the problems of (3) and (4), instead of titanium oxide having a high refractive index, there have been no solution means for performing sufficient effects found yet.
At this conventional technological level, there are actually often used niobium oxide and tantalum oxide having a refractive index somewhat lower than that of titanium oxide, as a high refractive material in an optical multi-layer film. In this case, an optical design for obtaining necessary optical properties is difficult since the refractive index is smaller than that of titanium oxide.
In optical elements required to manifest desired properties only in an extremely precisely restricted wavelength range, such as an optical filter used in demultiplexer and multiplexer which are essential parts as a module for optical communications of a wavelength multiplex mode, the fact that the above-mentioned problems have not been solved is an extremely important problem industrially.